The present invention relates to interference cancelling systems and more particularly to techniques for improving cancellation in a frequency agile radar system.
Generally, in signal receiving systems such as radar systems, signals received in the side-lobes of the radar antenna interfere with the isolation of desired signals received in the main lobe of the antenna. Accordingly, to isolate the main lobe signals, side-lobe cancellers have been used to cancel the side-lobe interference in the main radar antenna as exampled by U.S. Pat. No. 3,202,990.
Conventionally, such side-lobe canceller systems have employed a plurality of adaptive canceller loops using offset auxiliary interference signals to provide cancellation in the main radar signal. In some similarly known systems, baseband adaptive loops have used quadrature hybrid circuits to separate the auxiliary interference signal into I and Q components. When each of the I and Q components is coupled to form its own adaptive loop in parallel with the other, the auxiliary interference signals can again be subtracted to reduce interference in the main radar signal. In either case, however, the adaptive loops achieve cancellation by subtracting a properly weighted auxiliary interference signal from the signal in the main radar antenna, where the proper weights are derived in a correlation process operating over a narrow band of frequencies centered about a fixed frequency.
It has been found that while such known systems have been highly successful in achieving good cancellation in most instances, limitations are encountered when the radar frequency is changed on a pulse to pulse basis (frequency agility) as contrasted with conventional fixed frequency radars. This is due primarily to the finite settling time required by the adaptive canceler loops to develop a proper weight for good cancellation at the radar frequency. Therefore, while the frequency agile radar system is highly desirable because of its decreased susceptibility to hostile detection and countermeasure techniques, the limitations on cancellation, flexibility, and range, imposed by the loop settling times, significantly curtails the effectiveness of side-lobe cancellers in highly sophisticated radar systems, particularly in a barrage jamming environment.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an improved and more versatile interference cancelling system for use with modern radar systems.